1. Field of the Invention
The present invention relates to a method for forming a thin film which is used as conductor in a semiconductor integrated circuit and more particularly to a method for forming a planarized thin film over an undercoat film having a rough surface which is not planarized.
2. Description of the Prior Art
So far, a vacuum evaporation or sputtering process has been widely used to form a thin metal film which is used as conductor in a semiconductor integrated circuit. However, the surfaces of a substrate over which thin metal films are formed are generally not planar, but are rough. That is, the surfaces have small ridges and valleys or are convex and concave. As a result, when a thin film is formed over the rough surface of such a substrate by the vacuum evaporation or sputtering process, a shape of a thin film covering the rough surface is deteriorated. For instance, an overhanging thin film is formed at a ridge so that a micro-crack occurs at a step of the ridge to result in a wiring breakdown such as disconnection.
In order to solve the above-described problems, it is required that a thin metal film completely fills ridges and valleys of the substrate to planarize the rough surface of the substrate as much as possible. In order to satisfy the above-described requirement, we can consider the following two processes. One process is such that after a thin metal film has been formed over a rough surface of a substrate to poorly cover the rough surface, defective portions of the thin metal film thus poorly covering the rough surface are remedied by some methods. The other process is such that a planarized thin film is so formed that its planarized surface covers uniformly the rough surface.
However, there has not been proposed yet the former process which is satisfactory in practice. As the latter process, there is a technique which is a so-called "bias sputtering method" such as a proposal by Homma et al. in Journal of Electrochemical Society, Vol. 132, (1985), pp. 1466-1472, "Planar Deposition of Aluminum by RF/DC Sputtering with RF Bias". In the bias sputtering method, a material of a thin film is sputtered to form a thin film over the rough surface of a substrate under a condition that a DC or AC bias voltage such as a voltage in a range of -100 V through -500 V is applied to the substrate According to the conventional bias sputtering method, the formation of a thin film proceeds while a part of the thin film deposited over the substrate is being etched out.
That is, as shown in FIG. 1, the bias sputtering method utilizes the fact that an etching rate of a thin film is dependent upon an ion incident angle in order to planarize the surface of the deposited thin film, as will be described in detail hereinafter.
The planarization process will be explained in detail with referring to FIGS. 2A-2E. While a thin film formation period is short, an unsatisfactory Al film 101 with poor planarization is formed over a substrate 100 having convex portions 102 as shown in FIG. 2A. Thereafter, in a little while, the inclined surfaces of the Al thin film 101 are etched faster than the horizontal surfaces thereof, and at the same time, Al is deposited over the inclined and horizontal surfaces, so that the covering shape of the thin film being deposited varies as shown in FIGS. 2B and 2C. When the formation of the thin film is further carried out, the etching rate at the inclined surfaces is faster than that at the horizontal surfaces parallel to the surface of the substrate 100, so that the inclined surfaces recede toward the middle portions of the projections 102, as shown in FIG. 2D. In this case, while the deposited thin film is etched, a new thin film is formed by the deposition of sputtered atoms flying from a target, so that the thickness of the thin film 101 is not so extremely thin. As the time further elapses, the inclined surfaces over the projections 102 further recede toward the middle portions of the projections 102 and finally the inclined surfaces which are receding from both sides disappear at the midddle portions of the projections 102, so that the thin film 101 has a planarized surface, as shown in FIG. 2E.
The above-described bias sputtering method, however, has the following fatal defects.
Firstly, the rate at which a new thin film is formed by the deposition of sputtered atoms from the target must be lower than the rate at which the inclined surfaces recede or disappear, so that a planarized surface is obtained. As a result, there arises the problem that it takes a long time before the surface of the thin film is fully planarized. According to a conventional magnetron sputtering process, the deposition rate of aluminum is about 1 .mu.m/min, while according to the above described bias sputtering process, the deposition rate is about 20 nm/min. That is, the deposition rate of the bias sputtering process is 50 times as slow as the deposition rate of the magnetron sputtering process.
Secondly, as is clear from the above-described explanation with reference to FIGS. 2A-2E, when the width of a projection is narrow, the surface of a thin film can be planarized within a relatively short time, but when the width of a projection is wide, it takes a longer time before the surface of a thin film deposited is planarized. As a result, when the surface pattern of an undercoat or substrate has projections in various sizes, the thickness and the shape of a thin film deposited over the undercoat or substrate are dependent upon the surface pattern of the undercoat or substrate. More particularly, the thickness of the thin film varies from one pattern projection to another and the surface on a large pattern projection is not sufficiently planarized. Due to this dependency of the surface of the thin film on the surface pattern of the substrate, the thin film remains partially unetched in the succeeding etching step, so that a yield of the finished product becomes low.
In addition, according to the bias sputtering process, accelerated ions are made to impinge against a surface of a substrate, so that sputter etching proceeds at the same time that a thin film is deposited over the surface of the substrate. As a result, during the initial time that a thin film is not sufficiently deposited over the surface of the substrate, the elements of the substrate which have been sputter-etched are mixed into the thin film being deposited and accelerated ions are also injected into the thin film, so that a purity of the deposited thin film is lowered. When ions are impinged against the substrate surface with a higher acceleration energy, the structure of the deposited thin film is varied and accordingly a high-quality thin film cannot be formed continuously.
FIG. 3 is a photograph taken by a scanning type electron microscope, illustrating the surface condition when a thin aluminum film is formed over the surface of an SiO.sub.2 film by the bias sputtering process with a higher acceleration energy. It is seen that the crystal grain growth of aluminum is adversely restrained by oxygen and silicon emitted from the substrate so that column crystals in which crystal grains are spaced apart from each other are grown. As a result, the aluminum film thus deposited exhibits infinite resistance.
Because of the defects described above, the bias sputtering process has not been satisfactorily used in practice to form a thin metal film.